Purpose: Various design concepts have been adopted in cervical disc prostheses, including sliding articulation and standalone configuration. This study aimed to evaluate the biomechanical effects of the standalone U-shaped configuration on the cervical spine.
Methods: Based on an intact finite element model of C3-C7, a standalone U-shaped implant (DCI) was installed at C5-C6 and compared with a sliding articulation design (Prodisc-C) and an anterior fusion system. The range of motion (ROM), adjacent intradiscal pressure (IDP) and capsular ligament strain were calculated under different spinal motions.
Results: Compared to the intact configuration, the ROM at C5-C6 was reduced by 90% after fusion, but increased by 70% in the Prodisc-C model, while the maximum percentage change in the DCI model was 30% decrease. At the adjacent segments, up to 32% increase in ROM happened after fusion, while up to 34% decrease occurred in Prodisc-C model and 17% decrease in DCI model. The IDP increased by 11.6% after fusion, but decreased by 5.6 and 6.3% in the DCI and Prodisc-C model, respectively. The capsular ligament strain increased by 147% in Prodisc-C and by 13% in the DCI model. The DCI implant exhibited a high stress distribution.
Conclusions: Spinal fusion resulted in compensatory increase of ROM at the adjacent sites, thereby elevating the IDP. Prodisc-C resulted in hyper-mobility at the operative site that led to an increase of ligament force and strain. The U-shaped implant could maintain the spinal kinematics and impose minimum influence on the adjacent soft tissues, despite the standalone configuration encountering the disadvantages of high stress distribution.